1. Field of the Invention
The present invention relates to a position transducer used in the fields of machine tools, industrial machinery, etc. to detect a position changed due to a rectilinear movement, rotational movement or the like.
This application claims the priority of the Japanese Patent Application No. 2002-059265 filed on Mar. 5, 2002, the entirety of which is incorporated by reference herein.
2. Description of the Related Art
The position transducers used in th fields of machine tools, industrial machinery, etc. to detect a position changed due to a rectilinear movement or the like are known from the disclosure in the Japanese Unexamined Application Publication Nos. 99564 of 1985, 177019 of 1988, 152314 of 1989, etc.
Referring now to FIG. 1, there is schematically illustrated in the form of a block diagram a conventional position transducer with one absolute track. The position transducer is generally indicated with a reference 120. As shown, the position transducer 120 is composed of a one-track absolute track 121 and an ABS head 124 including n absolute track detectors 1231, 1232, . . . , 123n. It should be noted that in the following explanation, the absolute track will be referred to as xe2x80x9cABS trackxe2x80x9d and the ABS track detector be referred to as xe2x80x9cABS detectorxe2x80x9d.
The ABS track 121 is formed from a pattern in which micro-areas each represented by xe2x80x9c0xe2x80x9d and ones each represented by xe2x80x9c1xe2x80x9d are laid in array according to an n-order cyclic code. The micro-area represented by xe2x80x9c0xe2x80x9d and one represented by xe2x80x9c1xe2x80x9d are different in physical property from each other. For example, for the position transducer 120 to magnetically detect the position of the ABS head 124, the micro-areas xe2x80x9c1xe2x80x9d are magnetized but the micro-areas xe2x80x9c0xe2x80x9d are not.
The ABS head 124 is movable longitudinally of the ABS track 121 as indicated by an arrow Y in FIG. 1. Each of the ABS detectors 1231 to 123n is provided in a position opposite to the ABS track 121. Also, when the micro-areas are laid at a pitch xcex, the ABS detectors 1231 to 123n, are spaced a distance xcex corresponding to the micro-area pitch xcex from each other.
In the aforementioned position transducer 120, since the ABS track 121 is formed as the pattern in which the micro-areas each represented by xe2x80x9c0xe2x80x9d and ones each represented by xe2x80x9c1xe2x80x9d are laid in array according to the n-order cyclic code, as the ABS head 124 moves in relation to the ABS track 121, the ABS head 124 will detect ABS values all different from each other irrespectively of the fact that only the ABS track 121 is included in the position transducer 120. Therefore, the position transducer 120 can detect the position of the ABS head 124 in relation to the ABS track 121 on the basis of the ABS values detected by the ABS head 124.
The position transducer 120 can have the ABS track 121 reduced in size in a direction indicated by an arrow Z in FIG. 1 because it has only one ABS track 121. That is, the position transducer 120 can be designed smaller.
In the position transducer 120 having only the ABS track 121, when the position of the ABS head 124 varies, signals outputs from two or more of the ABS detectors 1231 to 123n will possibly be switched from one to another. Specifically, signal outputs from two or more of the ABS detectors 1231 to 123n are switched from xe2x80x9c0xe2x80x9d to xe2x80x9c1xe2x80x9d or vice versa.
It is difficult to provide all the ABS detectors 1231 to 123n having quite the same capability of detecting a physical property and outputting a signal. Also it is difficult to provide all the micro-areas spaced at quite the same pitch xcex (as shown in FIG. 1) from each other when producing the ABS track 121.
Therefore, when each of the ABS detectors 1231 to 123n is opposite to near the edge of a micro-area, it is difficult for the ABS head 124 to accurately detect a code from the ABS track 121 because the ABS detectors 1231 to 123n output signals at different times, respectively.
Also, when each of the ABS detectors 1231 to 123n is opposite to near the edge of a micro-area, the physical property to be detected is influenced by an adjacent micro-area. For example, when the ABS detector 1231 detects a magnetized micro-area near the edge and an adjacent micro-areas is not magnetized, it will only detect a weak magnetic field and thus output a small signal.
Namely, when each of the ABS detectors 1231 to 123n is opposite to near the edge of a micro-area, the position transducer 120 can hardly detect, with accuracy, the position of the ABS head 124 in relation to the ABS track 121.
Position transducers capable of detecting the position of the ABS head in relation to the ABS track without error include two types which will be described below.
The first one of the two types of position transducers is disclosed in the Japanese Unexamined Application Publication No. 1990-21216, Japanese Patent No. 2679207 and Japanese Unexamined Application Publication No. 1989-152314, respectively. This position transducer detects no signal output from each of the ABS detectors that is opposite to near the edge of a micro-area but detects signal output from each of the ABS detectors that is opposite to near the center of the micro-area (which area will be referred to as xe2x80x9cstable areaxe2x80x9d hereunder).
FIG. 2 is also a block diagram of a variant of the aforementioned conventional position transducer. This conventional position transducer is generally indicated with a reference 140. As shown, in the position transducer 140, an incremental track 142 whose physical property changes in a constant cycle is juxtaposed with the ABS track 121. Also, the position transducer 140 includes, in addition to the ABS head 124, an incremental track head 144 having two incremental track detectors 1431 and 1432.
The position transducer 140 is adapted such that judging from a signal output from the incremental track head 144 that each of the ABS detectors 1231 to 123n is opposite to the stable area, it detects signal outputs from the ABS detectors 1231 to 123n.
When each of the ABS detectors 1231 to 123n is not opposite to any stable area, however, the aforementioned position transducer 140 cannot detect any position of the ABS head 124 in relation to the ABS track 121.
Therefore, for example, to detect the position of the ABS head 124 in relation to the ABS track 121 the moment the power is applied to the position transducer 140, each of the ABS detectors 1231 to 123n have to be opposite to the stable area at that moment. If none of the ABS detectors 1231 to 123n are opposite to their respective stable areas, the position transducer 140 cannot detect any position of the ABS head 124 in relation to the ABS track 121 before the ABS head 124 is moved until each of the ABS detectors 1231 to 123n moves into the stable area. Namely, the position transducer 140 can hardly detect the position of the ABS head 124 in relation to the ABS track 121 the movement it is applied with the power.
The second type of position transducer is disclosed in the Japanese Patent Nos. 2571393, 2571394, 3063044 and 3103266. The positional transducer includes n sets of ABS detectors, each set including two ABS detectors. In this position transducer, signal output from one of the two ABS detectors that is opposite to the stable area is used to detect an ABS value.
In this position transducer, however, the ABS head includes 2n ABS detectors to detect an n-bit code indicating an ABS value. That is, the position transducer will have a larger number of ABS detectors for the number of bits in a code the ABS head detects. Since many ABS detections are made in the position transducer, the ABS detectors will possibly be large and corresponding addition of wires and the like will possibly cause the circuit to be more complicated. Therefore, this position transducer can hardly be designed compact. Also, since this position transducer is produced through many steps, it is difficult to produce it with less costs.
It is therefore an object of the present invention to overcome the above-mentioned drawbacks of the related art by providing a position transducer capable of accurately detecting an n-bit code by an absolute track head including n absolute track detectors even when it includes one absolute track and also capable of detecting the position of the absolute track head in relation to the absolute track independently of the position of each absolute track detector.
The above object can be attained by providing a position transducer including according to the present invention:
an absolute track in which micro-areas each represented by xe2x80x9c0xe2x80x9d and those each represented by xe2x80x9c1xe2x80x9d when two types of micro-areas different in physical property from each other are represented by either of two codes xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d are laid in array to provide an n-bit code (n is a natural number);
an incremental track whose physical property varies in a constant cycle;
an absolute track head having n detectors laid in array oppositely to the absolute track and which is movable in relation to the absolute track to detect the physical property of the absolute track;
an incremental track head having detectors opposite to the incremental track to detect the physical property of the incremental track;
a sorting means for binarizing the values of signal outputs from the absolute track detectors to xe2x80x9cHxe2x80x9d and xe2x80x9cLxe2x80x9d based on a first threshold h and also to xe2x80x9cHxe2x80x9d and xe2x80x9cLxe2x80x9d based on a second threshold l (h greater than 1) and then sorting the signal values into ones having been judged as being xe2x80x9cHxe2x80x9d or xe2x80x9cLxe2x80x9d both when the signal outputs are binarized based on the first threshold h and when they are binarized based on the second threshold l and ones having been judged as being xe2x80x9cLxe2x80x9d when they are binarized based on the first threshold h and as being xe2x80x9cHxe2x80x9d when they are binarized based on the second threshold l;
an interpolation means for segmenting, within a micro-area, the position of the absolute track head in relation to the absolute track given by the n-bit code on the basis of the signal output from the incremental track detector; and
a code deciding means for deciding the n-bit code based on the signal having been judged by the sorting means as being under than l or over h and the result of the segmentation by the interpolation means.